Oxidation resistant chromium alloy



United States Patent Ofiice 2,955,937 Patented Oct. 11, 1960 2 Claims.(Cl. 75-176) No Drawing.

This invention relates in general to high temperature oxidationresistant alloys and specifically to chromium base alloys possessinggood high temperature oxidation resistance,

It is an object of this invention to provide alloys with superiorresistance to oxidation at temperatures in excess of 1800 F.

A further object of this invention is to provide alloys with superiorcorrosion resistance, and good strength and ductility characteristics attemperatures in excess of 1800 F.

Yet another object of this invention is to provide chromium base alloyswhich will resist oxidation and corrosion at temperatures in excess of1800 F., and which will retain good strength and ductilitycharacteristics at such elevated temperatures.

For applications in air, the field of high temperature metallurgy is formost practical purposes limited to the utilization of iron, cobalt andnickel base alloys, and the use of even these materials under stress islimited to temperatures not in excess of 2000 F. The only other readilyavailable metals which might be expected to have useful strength andoxidation resistance above 2000 F. are chromium base alloys. Now, it isknown that high temperature, high strength structural chromium basealloys do exist, but the usefulness of such chromium base alloys hasbeen limited by the fact that the oxide film formed on the surface ofsuch an alloy does not afford the alloy much protection against furtheroxidation at high temperatures. The chromium oxide film formed when themetal is heated in air does afford some useful protection attemperatures up to 1700 -F., however, above this temperature attack issufliciently rapid to discourage use of the metal at such elevatedtemperatures. Certainly, the oxidation resistance of chromium at 2000 F.can not be compared favorably to that of iron or nickel alloyscontaining major additions of chromium or aluminum. To make thesituation even worse, unlike iron, cobalt and nickel, chromium alsoreacts with the nitrogen in air, even at temperatures below 1700 F. Themechanism of this reaction is such that nitrogen contamination is notlimited to a surface attack on the alloy, but extends deeply into thechromuim because of a rather rapid penetration of nitrogen into themetal by diflusion. The effect of such a nitrogen attack is theconversion of the chromium alloy from a ductile high strength structuralmetal to a brittle substance of practically no useful ductility. Priorto the research which resulted in the discovery which is the subject ofthis application, all of the known alloying metals which could be addedto chromium without impairing its useful metallic properties werestudied with the object of producing chromium base alloys that would beresistant to oxygen and nitrogen attack in air at elevated temperatures;but no useful alloys were discovered. Elements such as beryllium, boron,aluminum, titanium, zirconium and silicon which tend to improve theoxidation resistance of iron, cobalt, or nickel base alloys were Whollyinefiective for improving the high temperature stability of chromium.

Since aluminum proved to be ineffective for improving the oxidationresistance of chromium there was little reason to suspect that scandium,yttrium, or the rare elements, which also belong to the group III of theperiodic table would be effective. From what little was known ofchromium-base alloys containing the rare earths, it appeared that thesolid solubility of such elements in chromium would not be large andtherefore would not be too effective. Furthermore, from previouslyreported work on chromium base alloys containing cerium,.one of the rareearths, it was known that cerium was not eifective for preventingnitrogen absorption of the metal at high temperatures.

The eifect of small amounts of yttrium, and certain other rare earths,on chromium is therefore extraordinary, for although the solubility ofyttrium in chromium is not over 1 percent, additions in this amount aresufiicient to provide alloys which are stable in air to temperatureswell above 2000' F. and even above 2200 F. and in addition, these alloysare much stronger than iron, cobalt, or nickel base alloys and also haveexcellent ductility for structural applications.

During the research which led to the discovery of these oxidationresistant chromium alloys, a series of small arc melted chromium-yttriumbuttons were prepared. These alloy buttons, containing from 0.2 to 10percent yttrium, were heated in air at 2200 F. for hours. The specimenswere then examined metallographically, weight changes measured, andnitrogen contamination measured by chemical analysis. The data tabulatedin Table I shows the excellent stability of the chromium alloyscontaining yttrium as compared to pure chromium and some other chromiumbase alloys. The

unique stability of the chromium-yttrium alloys is apparent.

TABLE I Oxidation resistance of chromium base alloys [100 hrs., air,2,200 F.]

Additions, Hard- Nature of Oxidized Wt. Nitrogen w/o ness, Surface Gain,Pickup,

R percent p.p.m.

100% Or 43 Heavy scale 2. 7 2, 470

0.2 Y- 0. 4 370 0.5 Y 0. l 1.0 Y. 0.1 1.5 Y 0. l

2.5 Y 0.3 5 Y 0. 7 10 Y 0 2 A1. 0.9 5 A1- 2. 1 7.5 A 1. 1 10 Al 1.0do 1. 1 Completely 0x1 zed 3.7 do 2. 3 70 do 5.3

From this data it was found that as little as 0.2 weight percent yttriumis effective for improving the oxidation resistance of chromium,although from 1 to 1.5 weight percent yttrium appears to be the optimumamount of yttrium addition. As the yttrium content is increased beyond1.5 percent, the oxidation stability of the alloy is adversely alfected.While we do not wish to be restricted by theory, we believe that theexplanation for this loss of oxidation stability lies in the fact thatthe maximum solid solubility of yttrium in chromium is not much over 1percent. As a result, alloys containing more than 1.5 percent yttriumactually contain a second phase of the yttrium metal solid solution andthis second phase is probably not stable in air at elevatedtemperatures. Hence, the decrease in oxidation resistance of chromiumalloys containing more than 1.5 percent yttrium appears to be due tooxygen and nitrogen attack on this second phase.

Although the exactmeehanism by which the Yttrium. additions affordsprotection to chromium base .alloysis.

notrknown, and although, again, applicants do. not .wish tObeJimited, bytheory, we arerof the opinion that. yttrium in combination with chromiumis preferentially oxidizedv to form Y O which may or may not react Withchromium oxide and that the resultant film formed on the chromiumprobably is, impervious to oxygen or nitrogen. Chromiumalso appearsunable to difiuse through this film. v

Chromium base alloyscontaining yttrium when alloyed 'with other.elements such as. molybdenum, columbium, tungsten,.etc., retain the goodoxidation resistance. char-v acteristics of. the binarychromium-yttriumalloy. Some of; the rare; earth elements. which are verysimilarv to yttrium in metallurgical properties also .aflo'rd oxidationresistance to chromium. The rare earths differ greatly among themselves,withregard to. their mettalurgical monvolatile and stable in air. Thelatter metals, along with scandium and other rare :earths which aresimilar to yttrium may also be employed to provide high temperatureoxidation resistant chromium base alloys.

While this invention has been described with reference to specificembodiments thereof; it is not intended to limit the invention n n n 'wre by t qsce e f t appended claimsr I V I a We claimze .1. A binaryalloyoichromii1m-and yttrium consisting of from about 0.2 Weightpercentto about 2g5 .weight percent yttrium,"tl1e'balancebeing chromium.

2-. A in ryalloy offihrqmipmand. y trium consisting of from about lfiweightpercentto "about 155* weight percent yttrium,thfljbalance"beinggchrornium.

VN.Y., 1954, p. 334.

Gilbertet al.:- Malleable Chromium'and Its: Alloys, Bureau of Mines,Report of Investigations 4905; 1952, pp.- 12,. 14.. i V

1. A BINARY ALLOY OF CHROMIUM AND YTTRIUM CONSISTING OF FROM ABOUT 0.2WEIGHT PERCENT TO ABOUT 2.5 WEIGHT PERCENT YTTRIUM, THE BALANCE BEINGCHROMIUM.